1. Field of the Invention
The present invention relates to a semiconductor device and a photoelectric conversion apparatus using the device.
2. Related Background Art
Conventionally, wide variety of configurations and structures are used as the semiconductor device. Among them, a bipolar transistor (hereinafter, referred to as "BPT") has many advantages that high speed switching operation can be expected.
A DOPOSBPT (Doped Poly Silicon BPT), i.e., a BPT having a polysilicon emitter, is known as a BPT having a shallow junction and a high packing density.
FIG. 1 is a sectional view showing an example of a conventional BPT. Referring to FIG. 1, reference numeral 1 denotes a substrate made of silicon (Si); 2, an n.sup.+ -type buried region including material impurity making conductivity into n-type within Si; 3, an n.sup.- -type region having a low impurity concentration; 4, a p-type region including material (impurity) making conductivity into p-type serving as a base region; 5, an n.sup.+ -type region including n-type impurity serving as an emitter region; 6, an n-type region serving as a channel stopper; 7, an n.sup.+ -type region of high impurity concentration for reducing a collector resistance of a bipolar transistor; 101, 102, 103, and 104, insulating films for insulating the element, electrodes, and wirings; 200, an electrode made of a metal, silicide, polycide, or the like.
The substrate 1 has an n conductivity type upon doping of an impurity such as phosphorus (Ph), antimony (Sb), or arsenic (As), or a p conductivity type upon doping of an impurity such as boron (B), aluminum (Al), or gallium (Ga). The buried region 2 need not be necessarily formed. Boron (B), gallium (Ga), or aluminum (Al), and germanium (Ge) are doped in the base region 4. The emitter region 5 consists of polysilicon formed by low-pressure chemical vapor deposition (LPCVD).
The base region 4 consists of a mixed crystal of silicon (Si) and germanium (Ge). Both of Si and Ge are diamond type crystals and perfect solid solutions. Therefore, Si.sub.l-X Ge.sub.X is a perfect diamond type crystal for every X (0 to 1). A forbidden band width E.sub.g is about 1.1 eV for Si and about 0.7 eV for Ge.
FIG. 2 is a graph showing a relationship between a mixed crystal ratio X and the forbidden band width E.sub.g of Si.sub.l-X Ge.sub.X. Referring to FIG. 2, the abscissa represents the mixed crystal ratio X, the ordinate represents the forbidden band width E.sub.g, a reduction width .DELTA.E.sub.C at a conduction band side, and a reduction width .DELTA.E.sub.V at a valence band side. As is apparent from FIG. 13, in Si.sub.l-X Ge.sub.X, most of a band gap reduction occurs in a valence band. This is very convenient for the hetero BPT because injection of electrons from an emitter to a base is not prevented.
In another configuration, the BPT of this type comprises a polysilicon emitter region. A silicon oxide having a thickness of 10 to 20 .degree. .ANG. is formed between the polysilicon emitter region and an emitter region formed by diffusing an impurity from polysilicon to single-crystal silicon.
FIG. 3 is a schematic sectional view showing a conventional BPT. Referring to FIG. 3, reference numeral 1 denotes a substrate; 2, an n.sup.+ -type buried region; 3, an n.sup.- -type region having a low impurity concentration; 4, a p-type region serving as a base region; 5, an n.sup.+ -type region serving as an emitter region; 6, an n-type region serving as a channel stopper; 7, an n.sup.+ -type region for reducing a collector resistance of the bipolar transistor; 8, silicon oxide regions formed in the single-crystal emitter region 101, 102, 103, and 104, insulating films for isolating the element, electrodes, and wirings; and 200, electrodes made of a metal, a silicide, a polycide, or the like.
The substrate 1 has an n conductivity type upon doping of an impurity such as phosphorus (Ph), antimony (Sb), or arsenic (As), or a p conductivity type upon doping of an impurity such as boron (B), aluminum (Al), or gallium (Ga). The buried region 2 need not be necessarily formed. The n.sup.- -type region is formed by epitaxial techniques. Boron (B), gallium (Ga), or aluminum (Al), and germanium (Ge) are doped in the base region 4. The emitter region 5 consists of polysilicon formed by low-pressure chemical vapor deposition (LPCVD).
This BPT, i.e., the BPT having the single crystal emitter region and the silicon oxide formed between the single crystal emitter region and the base region, has an advantage in that the oxide film can increase a current gain of the BPT.
Analytically theoretical expressions will be described with reference to FIG. 4 to explain the reason why the oxide film can increase the current gain of the BPT.
FIG. 4 shows a diagram showing potentials in the direction of depth of the cross section along the line A--A' in FIG. 3 in a normal operation. Referring to FIG. 4, reference numeral W.sub.E denotes a thickness of an emitter neutral region; W.sub.B, a thickness of a base neutral-region. As shown in FIG. 4, since the oxide is formed between the emitter region and the base region, a potential barrier is present at a position of W.sub.E '.
In the conventional semiconductor device having the above structure, the base current consists of the following two components.
A diffusion current of positive holes flowing from the base to the emitter can be approximated as follows due to the presence of the potential barrier: EQU J.sub.Bl =(q.multidot.n.sub.i.sup.2 .multidot.D.sub.P /N.sub.E .multidot.L.sub.P) x tanh(W.sub.E '/L.sub.P)[exp(V.sub.BE / kT)-l] (1)
A recombination current of electrons injected from the emitter is represented as follows: ##EQU1##
A collector current is represented as follows: EQU J.sub.C =(q.multidot.n.sub.i.sup.2 .multidot.D.sub.n /N.sub.B .multidot.L.sub.n)[cosech(W.sub.B /L.sub.N)] x[exp(V.sub.BE /kT)-l] (3)
where q is a charge, n.sub.i is an intrinsic semiconductor charge density (Si), N.sub.E is an emitter impurity density, N.sub.B is a base impurity density, D.sub.P is a positive hole diffusion coefficient, D.sub.N is an electron diffusion coefficient, L.sub.P is a positive hole diffusion length (.perspectiveto.(D.sub.P .tau..sub.P).sup.1/2), L.sub.N is an electron diffusion length (.perspectiveto.(D.sub.N .tau..sub.N).sup.1/2), k is a Boltzmann constant, T is an absolute temperature, V.sub.BE represents base-emitter forward biased electrons, .tau..sub.P and .tau..sub.N are minority carrier lifetimes of the positive holes and the electrons.
In equation (1), when W.sub.E ' is decreased, J.sub.Bl is reduced. Therefore, a current gain can be increased: EQU h.sub.FE =J.sub.C /(J.sub.Bl +J.sub.B2) (4)
FIG. 5 is a schematic sectional view showing a different type of BPT. Referring to FIG. 5, reference numeral 1 denotes a substrate; 2, an n.sup.+ -type buried region; 3, an n.sup.- -type region having a low impurity concentration; 4, a p-type region serving as a base region; 5, an n.sup.+ -type region serving as an emitter region; 6, an n-type region serving as a channel stopper; 7, an n.sup.+ -type region for reducing a collector resistance of the bipolar transistor; 101, 102, 103, and 104, insulating films for insulating the element, electrodes, and wirings; and 200, electrodes made of a metal, a silicide, a polycide, or the like.
The substrate 1 has an n conductivity type upon doping of an impurity such as phosphorus (Ph), antimony (Sb), or arsenic (As), or a p conductivity type upon doping of an impurity such as boron (B), aluminum (Al), or gallium (Ga). The buried region 2 need not be necessarily formed. The n.sup.- -type region 3 is formed by epitaxial techniques. Boron (B), gallium (Ga), or aluminum (Al), and germanium (Ge) are doped in the base region 4. The emitter region consists of polysilicon.
In the conventional semiconductor device having the above structure, the base current consists of the following two components.
A diffusion current of positive holes flowing from the base to the emitter can be approximated as follows due to the presence of the potential barrier: EQU J.sub.Bl ={(q.multidot.n.sub.i.sup.2 .multidot.D.sub.p)/(N.sub.E .multidot.L.sub.p)}x cosh(W.sub.E /L.sub.p) {exp(V.sub.BE /kT)-1} (1)
A recombination current of electrons injected from the emitter is represented as follows: ##EQU2##
A collector current is represented as follows: EQU J.sub.C ={(q.multidot.n.sub.i.sup.2 .multidot.D.sub.n)/(N.sub.B .multidot.L.sub.n)}x cosech(W.sub.B /L.sub.n) {exp(V.sub.BE /kT)-1} (3)
where q is a charge, n.sub.i is an intrinsic semiconductor charge density (Si), N.sub.E is an emitter impurity density, N.sub.B is a base impurity density, D.sub.P is a positive hole diffusion coefficient, D.sub.N is an electron diffusion coefficient, L.sub.P is a positive hole diffusion length (.perspectiveto.(D.sub.P .tau..sub.P).sup.1/2), L.sub.N is an electron diffusion length (D.sub.N .tau..sub.N).sup.1/2), k is a Boltzmann constant. T is an absolute temperature, V.sub.BE represents base-emitter forward biased electrons, W.sub.B is a thickness of the base, and W.sub.E is a thickness of the emitter. Note that .tau..sub.P and .tau..sub.N are minority carrier lifetimes of the positive holes and the electrons.
In the above conventional BPT, however, a point defect as an electrical recombination center or dislocation caused by a lattice defect occurs in an interface between the Si crystal forming the emitter region and the Si.sub.l-X Ge.sub.X eutectic forming the base region. Therefore, a defect occurs near an emitter-base junction or between the base and the collector to increase a base current of the BPT, thereby reducing a current gain h.sub.FE.
An influence of the point defect or dislocation on h.sub.FE is significantly increased at a low current side of a collector current, and h.sub.FE is reduced close to one or below one in some cases.
This will be described in detail below.
The point defect or dislocation at the interface between the Si crystal or Si.sub.l-X Ge.sub.X mixed crystal is caused by a difference in lattice coefficients between Si and Si.sub.l-X Ge.sub.X.
The lattice coefficient of Si is d.sub.Si =5.43086.ANG., while that of Ge is d.sub.Ge =5.65748.ANG., i.e., a lattice coefficient difference is about 4%. Therefore, lattice coefficient values of the Si crystal and the Si.sub.l-X Ge.sub.X mixed crystal are different. For this reason, a stress occurs in the interface between the two crystals to partially cut a chemical combination between elements at the interface. This is a so-called point defect. When the point defect significantly occurs, dislocation is caused.
A constant relationship is present between the mixed crystal ratio X of Ge and the thickness of a layer in which no dislocation occurs. FIG. 6 is a graph showing this relationship. Note that this data is obtained when Si.sub.l-X Ge.sub.X is deposited on an Si substrate by the MBE method. Since growth is performed at 510.degree. C., a transition region from Si.sub.l-X Ge.sub.X to Si is very thin.
In a layer having a homogeneous mixed crystal composition X, dislocation occurs in the interface if the thickness is not less than that of a hatched region shown in FIG. 6.
Next, an impurity concentration of the emitter region 5 in the conventional BPT and the conventional DOPOSBPT falls within the range of 10.sup.19 to 10.sup.21 cm.sup.-3 ; an impurity concentration of the base region, 10.sup.16 to 10.sup.18 cm.sup.-3 ; an impurity concentration of the collector region; about 10.sup.14 to 10.sup.16 cm.sup.-3.
In such a BPT, since the impurity concentration of the emitter region is higher (10.sup.19 cm.sup.-3 or more), narrowing of the band gap occurs, and injection efficiency of carriers from the emitter to the base is degraded (i.e., a current gain h.sub.FE is decreased).
When an impurity concentration of a semiconductor is increased, a free carrier density is increased (i.e., minority carrier mobility is gradually decreased, and the band gap (forbidden gap width) is decreased). When the impurity density is 10.sup.17 to 10.sup.18 or more, band tailing occurs from the ends of the conduction and valence bands in an n- or p-type semiconductor.
FIGS. 7 and 8 are views showing band structures in semiconductors. Energy is plotted along the ordinate and a state density n(E) (i.e., the number of carriers per unit volume) is plotted along the abscissa in each of FIGS. 7 and 8. FIG. 7 is a view showing a band structure of an n-type semiconductor having a low impurity density. In this case, an n-type donor level is separated from a conduction band. However, FIG. 8 shows a semiconductor containing a high impurity density, and the width of a donor level is increased. The donor level becomes a donor band whose energy is coupled to that of the conduction band unique to the semiconductor. That is, as shown in FIG. 8, a degenerate conduction band is formed. As a result, band-end tailing occurs, and the band gap is changed from E.sub.g to E.sub.g ', thus causing band narrowing of .DELTA.E.sub.g =E.sub.g -E.sub.g '.
Band narrowing of the n-type semiconductor has been described in FIG. 8. Similar band tailing occurs on the side of the valence band in a p-type semiconductor having a high impurity concentration, and narrowing of a band width (prohibition band) E.sub.g occurs.
A band-narrowing value is approximated as follows: EQU .DELTA.E.sub.g ={3q.sup.2 /(16.pi..epsilon..sub.s)}{(q.sup.2 .multidot.N)/(.epsilon..sub.s kT)}.sup.1/2 ( 4)
where q is a charge, e is a semiconductor dielectric
s constant, k is a Boltzmann constant, T is an absolute temperature, and N is an impurity density.
A band-narrowing value of Si at room temperature is given as follows: EQU .DELTA.E.sub.g =22.5(N/10.sup.18).sup.178 meV (5)
For example, if N=10.sup.18 cm.sup.-3, then .DELTA.E.sub.q =22.5 meV.
FIG. 9 is a graph showing results of band-gap narrowing width calculations by using equation (5) when boron (B) is doped in Si in a high concentration. The impurity density (cm.sup.-3) of the impurity is plotted along the abscissa in FIG. 9, and the narrowed width .DELTA.E.sub.g (meV) of the band gap is plotted along the ordinate in FIG. 9.
An essential carrier density n.sub.i '.sup.2 upon occurrence of band-gap narrowing is given as follows as compared with the carrier density without narrowing: EQU n.sub.i '.sup.2 =n.sub.i.sup.2 esp(.DELTA.E.sub.g /kT) (6)
Since W.sub.B &lt;L.sub.n and J.sub.Bl &gt;J.sub.B2 are established, h.sub.FE is approximated as ##EQU3## The current gain can be expressed when only band-gap narrowing is taken into consideration: EQU h.sub.FE .perspectiveto.h.sub.FE0 .multidot.esp{(.DELTA.E.sub.gb -.DELTA.E.sub.ge)/kT} (8)
where h.sub.FE0 is a current gain without band-gap narrowing.
The conventional BPT had an emitter impurity concentration of about to 10.sup.21 cm.sup.-3, and a base impurity concentration of about 10.sup.16 to 10.sup.18 cm.sup.-3. For this reason, .DELTA.E.sub.ge is large, and .DELTA.E.sub.gb is almost zero. Therefore, the current gain h.sub.FE is smaller than the designed value.
When the current gain h.sub.FE is to be increased in a shallowed BPT, the impurity concentration of the base must be increased. When the impurity concentration of the base is, however, increased, the base-emitter breakdown voltage is decreased. In addition, the base-emitter capacitance is undesirably increased.
In a photoelectric transducer apparatus using a conventional BPT, a frequency f.sub.T is only about 1 GHz. The photoelectric transducer apparatus of this type is incompatible with HD (high Division; an area sensor coping with high vision).
To the contrary, as a conventional method of increasing the current gain h.sub.FE, a silicon oxide film having a thickness of 10-20 .ANG. has been formed between emitter region and base region. Such BPT can prevent hall from introducing from base region into emitter region due to a potential barrier formed within a valence band by an oxide film at an interface between the base and emitter. Therefore, the advantage that the current gain can be increased is obtained.
FIG. 10 shows a potential during a normal operation at A--A' sectional area wherein a silicon oxide is formed between the emitter and base regions of semiconductor device in FIG. 6. In the drawing, W.sub.E denotes a thickness of the emitter neutral area, W.sub.B denotes a thickness of the base neutral area. Further, as shown in the drawing, since a silicon oxide is formed between the emitter and base regions, a potential barrier is formed at a position of W.sub.E.
However, since the potential barrier due to the oxide film is formed not only at the conduction band but also at the valence band, it would be an obstacle against an electron stream as a carrier in the emitter, thereby emitter resistance is formed. Therefore, also it would be an obstacle against a collector current I.sub.c stream. It would be a cause of inclining the current property of current gain h.sub.FE, i.e. h.sub.FE depends upon base-emitter voltage V.sub.BE.
It is also difficult to obtain equal contents of oxygen in oxide films. The oxygen contents vary depending on individual BPTs, and therefore variations in BPT characteristics occur, resulting in inconvenience.
In particular, the characteristic variations of the individual BPTs are very important when they occur in a linear IC, a line sensor, and the like.